1. Field of the Invention
The present invention relates to a process for the catalytic conversion of hydrocarbons into aromatic compounds to be used for the reforming of gasolines and the production of aromatics.
In more precise manner, it relates to a process of this type using as catalyst a catalyst which comprises a matrix consisting of .eta. transition alumina or .gamma. transition alumina, or a mixture of these two aluminas, at least one doping metal chosen from the group made up of alkali and alkaline-earth metals, at least one halogen, at least one noble metal and at least one promoter metal.
2. Description of the Background
Catalyst reforming is a process which can be used to obtain improved octane ratings of petroleum fractions, in particular of heavy distillation gasoline by conversion of n-paraffins and naphthenes into aromatic hydrocarbons.
Catalyst reforming therefore entails the conversion firstly of C.sub.7 -C.sub.10 n-paraffins into aromatics and light paraffins, and secondly of C.sub.7 -C.sub.10 naphthenes into aromatics and light paraffins. These reactions are illustrated in particular by the conversion through dehydrogenation of cyclohexanes and dehydroisomerization of alkyl cyclopentanes to give aromatics, for example methyl cyclohexane giving toluene, and by the conversion through cyclization of n-paraffins into aromatics, for example n-heptane giving toluene.
During catalytic reforming, cracking reactions of heavy n-paraffins into light paraffins also take place, leading in particular to C1-C4 products, chiefly propane and isobutane: these reactions are detrimental to the yield of reformed product.
Finally, coke formation also takes place through condensation of aromatic nuclei to form a solid product rich in carbon which is deposited on the catalyst.
These reforming catalysts are extremely sensitive, in addition to coke, to various poisons likely to deteriorate their activity: in particular sulphur, nitrogen, metals and water.
When coke is deposited on the surface of the catalyst it leads to loss of activity in the course of time which gives rise to higher operating temperatures, a lower yield of reformed product and a higher yield of gas.
On this account, and taking into consideration the regeneration of the catalyst, the process of catalytic reforming can be implemented in two different manners: in semi-regenerative or cyclical manner and in continuous manner. In the former, the process is carried out in a fixed bed, and in the latter in a mobile bed.
In the semi-regenerative process, to offset the loss of activity of the catalyst, the temperature is gradually increased then the installation is stopped to proceed with regenerating the catalyst by eliminating the coke. In cyclical reforming which is in fact a variant of the semi-regenerative process, the installation comprises several reactors in series and each is put out of operation in turn, the coke deposits are eliminated from the catalyst placed out of circuit and the catalyst is regenerated while the other reactors remain in operation.
In continuous reforming, the reactors used are mobile bed reactors operating at low pressure (less than 15 bars), which allows for considerably improved yields of reformed product and hydrogen by promoting aromatization reactions to the detriment of cracking reactions, coke formation on the other hand being greatly accelerated. The catalyst passes through the reactors, then through a regenerating section.
On account of the chemical reactions that take place during reforming processes, a bifunctional catalyst must be used which combines two types of activity namely the hydrogenating-dehydrogenating activity of a metal, in particular a noble metal such as platinum, possibly in conjunction with other metals such as rhenium or tin, so-called promoter metals, this metal being deposited on the surface of a porous matrix. This matrix of alumina contains a halogen, preferably a chloride, which provides the necessary acidic function for isomerizations, cyclizations and cracking reactions.
The matrices generally used are chosen from among the refractory oxides of the metals of groups II, II and IV of the periodic table of elements. Aluminium oxide with the general formula Al.sub.2 O.sub.3.n.sub.2 O is most frequently used. Its specific surface area lies between 150 and 400 m.sup.2 /g. This oxide in which n lies between 0 and 0.6 is conventionally obtained by controlled dehydration of hydroxides in which 1.ltoreq.n.ltoreq.3. These amorphous hydroxides are themselves prepared by precipitation of aluminium salts in an aqueous medium by alkali salts. Precipitation and maturing conditions determine several forms of hydroxides, the most common being boehmite (n=1), gibbsite and bayerite (n=3). Depending upon hydrothermal treatment conditions, these hydroxides give several oxides or transition aluminas. Their forms are .rho., .gamma., .eta., .chi., .theta., .delta., .kappa. and .alpha. which distinguish themselves chiefly through the organization of their crystalline structure. During heat treatments, these different forms are likely to inter-evolve in accordance with a complex filiation system which is dependent upon operating conditions. The .alpha. form, which has a specific surface area and acidity in the region of zero, is the most stable at high temperatures. For catalysts, in particular for reforming catalysts, the .gamma. form of transition alumina is the form most often used as it offers a compromise between its acid properties and thermal stability.
As indicated above, the hydrogenation-dehydrogenation function is preferably provided by a noble metal from group VIII in the periodic table.
Numerous studies have especially examined the dehydrogenating function of these catalysts, and more specifically the type and method of introduction of the promoter metal added to the platinum. The chief effect of this second metal is to promote the dehydrogenating activity of platinum. In some cases, this second metal or promoter also produces the effect of limiting the dispersion loss of platinum atoms on the surface of the support. This dispersion loss is partly responsible for deactivation of the catalyst.
Among all the promoter metals examined, two metals hold a preponderant place rhenium and tin. It is these two metals which probably obtain the best promotion effects of platinum.
Therefore, the use of rhenium has in particular contributed to increasing the stability of the catalyst vis-a-vis its deactivation through the depositing of coke. This type of catalyst is most often used in fixed bed units. With this increase in stability, it has also been possible to increase the duration of the reaction cycles between two regenerations.
With tin, it has been possible to improve the performance of these catalysts when they are used at low pressure. This improvement in conjunction with their lower cracking activity has led to obtaining improved yields of reformed product especially in continuous regeneration processes operating at low pressure. Catalysts of this type containing rhenium, tin or even lead have been described in particular in U.S. Pat. No. 3,700,588 and U.S. Pat. No. 3,415,737.
For the conversion of hydrocarbons, a catalyst must offer a maximum level of activity but, in addition, must activate this conversion with the greatest possible selectivity. In particular, losses of hydrocarbons in the form of light products containing 1 to 4 atoms of carbon must be limited. The acid function is necessary for reactions producing aromatics and improving octane ratings. Unfortunately, this function is also responsible for cracking reactions which lead to the formation of light products. In consequence, it is evident that optimization of the quality of this acid function is of importance in order to further improve selectivity without however reducing the activity of the catalyst.
Catalysts must also be made more stable, that is to say resistant to coke poisoning.
Also, it has been seen that catalysts are used either in fixed bed processes or in mobile bed processes. In the latter, the catalysts undergo a high number of regenerations. These treatments whose action includes, amongst others, burning the coke deposited on the catalyst, are carried out at high temperatures in the presence of steam. Unfortunately, these conditions contribute to deterioration of the catalyst. It is therefore important to seek to increase the resistance of catalysts under such conditions.
Also, these catalysts are in the form of extrudates or beads whose size is sufficient to give relatively easy passage to reagents and gas products. The wear of these catalysts, in particular through friction in the mobile bed units, causes the formation of dust and finer grains. These finer grains disturb gas outflow and necessitate an increase in the entry pressure of the reagents, and even, in some cases, require the operation of the unit to be stopped. Moreover, in mobile bed units, the consequence of this gradual wear is to disturb the circulation of the catalyst and require frequent topping up with new catalyst.
A catalyst such as a reforming catalyst must therefore meet a high number of requirements or which some may appear to be contradictory. This catalyst must firstly offer the highest possible activity with which high yields can be obtained, but this activity must be combined with the greatest possible selectivity, that is to say that cracking reactions producing light products containing 1 to 4 carbon atoms must be limited.
Also, the catalyst must offer great stability against its deactivation through coke deposit: the catalyst must also offer excellent resistance to deterioration when subjected to the extreme conditions prevailing in the repeated regeneration operations it must undergo.
In the continuous reforming process using mobile bed reactors, as mentioned above, the catalysts are also subjected to intense gradual wear through friction leading to a substantial reduction in their specific surface area and to the formation of "fines" which are detrimental to the proper functioning of the installation. The catalysts currently available, while they may meet one or more of these conditions, do not fulfill all the requirements mentioned above.